How to make vertices level with a diagonal edge?
Method 1: Math


Align the 3D cursor with the center of your column. Set your pivot point to 3D cursor. Turn on Snap to Vertex using Active Element. Select the vertices on one half of your column.

Rotate the vertices (in your case about the Y-axis, in my case about the X-axis) to align with the top edge of the square. Be sure to note the amount your vertices rotate by in the bottom left corner. Calculate 1/cos(angle of rotation)

Scale the selected vertices with the axis about which you rotated locked (again Y in your case, X in mine. For you, S to scale, Shift+Y to lock Y axis) by the amount calculated. You can copy paste this amount.

Repeat for the other side, and you should be done.


Method 2: Shrinkwrap Modifier


Select the top edges of the square in the front. Duplicate these edges and separate them from the object.

Enter edit mode on the new object, which I have named 'Target' in my pictures. Extrude these edges along the Y-Axis (in my examples it is the X-Axis) so that they create the shape of your desired panels.

In your original object, create a new vertex group and add the vertices you wish move to this group.

Add a Shrinkwrap modifier to your original object. Set the target to your new object (in my case Target), and select your vertex group. Change the Mode to Project, and check the Z axis and both directions. Apply the modifier, and you should have what you want.